1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device used as a receiver circuit in a high speed light communication system.
2. Related Background Art
Typical receiver circuits of this type include trans-impedance type and high impedance type (NEC Technical Report, Vol. 39, No. 12, (1986) p. 94). The former has an advantage in that the dynamic range is wide when feedback is applied to an amplifying element. In the latter, a resistance R of a load resistor of a photo-sensing device is increased to reduce thermal noise generated by the load resistor, in order to increase sensitivity. In the past, the former trans-impedance type has been exclusively used and the latter high impedance type has been little used because for the trans-impedance type, it is easy to implement circuit by an integrated while the high impedance type is not suitable for implementation by an integrated circuit.
In the high impedance type receiver circuit, an equalizer for band compensation is required in a rear stage of the circuit because the band is limited by a cut-off frequency which is determined by a time constant (C.multidot.R.sub.L) where C is a capacitance of an input circuit including a junction capacitance of a photo-sensing device having a PN junction structure and an input capacitance of an amplifier circuit. The equalizer is constructed by connecting a capacitor and a resistor, in parallel and a time constant (C.sub.eq .multidot.R.sub.eq) must be equal to the time constant (C.multidot.R.sub.L), where C.sub.eq and R.sub.eq are the capacitance and resistance of the equalizer, in order to completely equalize a waveform distorted by band limitation by the photo-sensing device.
The junction capacitance of the photo-sensing device varies from type to type and there is variation even in the same type. As a result, the equalizer cannot be integrated in the same IC chip as that of the amplifier circuit but it should be off the chip and separately adjusted in accordance with the junction capacitance of the photo-sensing device which is separately combined. As a result, the number of parts and the number of production steps increase, which causes the reduction of productivity.